1. Field of the Invention
The Present invention relates to a method for manufacturing a liquid crystal (LC) display device and, in particular, to a method for manufacturing an active matrix type liquid crystal display device.
2. Description of the Related Art
An active matrix type liquid crystal display device includes active elements, such as TFT (Thin Film Transistor) and MIM (Metal Insulator Metal), to drive and control respective pixels. A liquid crystal display device having, for example, a TFT array will be explained below.
The liquid crystal display device includes first and second substrates and a liquid crystal sealed between these substrates. The first substrate includes a transparent substrate, made of ex. glass, and many address and data lines formed, in an intersecting fashion, over the transparent substrate with an insulating film formed therebetween. At the crosspoint of the address and data lines, a gate electrode which is formed integral with the address line is formed, and a semiconductor layer is formed over an insulating layer on the gate electrode. Drain electrode integral with the data line and source electrode are formed over the semiconductor layer such that they are located opposite to each other. In this way, a TFT element is constructed as a nonlinear active element. A transparent pixel electrode is formed over the insulating film and connected to a source electrode. The semiconductor layer and drain and source electrodes are covered with an insulating protective film. An orientation film is formed over the whole surface of the protective film and transparent pixel electrode.
The second substrate is located opposite to the first substrate and includes a transparent substrate made of, for example, glass and a transparent electrode and orientation film sequentially formed over the substrate. In a color display type LC display device, a color filter is formed under the transparent electrode.
The first and second substrates are sealed at their marginal portions, defining a predetermined clearance into which a liquid crystal is sealed.
In the TFT element of the aforementioned liquid crystal display device, the film forminmg step and photoetching step are repeated a plurality of times to provide a gate electrode, insulating film, semiconductor layer and drain and source electrodes.
The LC display device including the TFT array sometimes encounters "line defects". The line defects occur due to a connection breakage at the address and data lines and due to the short-circuiting caused between these lines or the short-circuiting at a spot where both the layers contact. These phenomena are caused by dust and incomplete photoetching steps effected during the manufacturing process. The interlayer short-circuiting may also be caused by a destruction by static electricity. In order to prevent such a destruction, as shown in FIG. 1, respective address lines 2 and respective data lines 4 are shorted by short-circuit lines 6, 8 in the conventional device to make the upper and lower conductor layers with an insulating film interposed therebetween equal in potential to each other. This method is proved effective to static electricity, but is never effective to a connection breakage occurring by other causes, such as short-circuiting between the associated layers.
Further, a plurality of TFT's 10 are connected to address and data lines (signal lines) and, if signal lines are broken for some reason or other, the corresponding TFT fails due to no voltage being applied to the TFT by the connection breakage. In other words, TFT's which are situated at a location distant from the connection breakage site with a voltage supply section as a reference are not turned ON. The connection breakage becomes a fatal defect for image display or a similar defect is also encountered due to a leakage of a signal current if adjacent signal lines are short-circuited.
Since such defects, unavoidably, occur during the manufacture of the LC display device, it is, therefore, necessary to eliminate them, if possible, in the earlier stage of the manufacturing process. It is important that the defects be eliminated by making an examination for a connection breakage, or short-circuiting, in the layered substrate.
It is conventional practice to form electrode pads on the electrode connected across the respective signal line so that checking may be made for examination purposes. The examination is made for the resistance of all the signal lines, that is, for a connection breakage, with the use of electrode pads. This method often uses a probe card for an enhanced operation efficiency whereby it is possible to measure the resistances of the signal lines in units of several tens of signal lines. Preparing such a probe card requires a very high accuracy and, therefore, there is a restriction to the number of signal lines measurable all at one time. Further, it takes a considerable time to make such measurement and measured values vary due to a wear or an injury of the probe. It is, therefore, difficult to make a check for short-circuiting between the adjacent signal lines because the address and data lines are arranged, in an intersecting fashion, with the insulating film formed therebetween and, in this case, are short-circuited so as to prevent dielectric breakdown by static electricity.